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Let \(\mathcal {C}\)  be a class of formal languages over an alphabet \(\Sigma \cup \lbrace \#\rbrace \)  such that each language in \(\mathcal {C}\)  has some associated finite description. Suppose \(\mathcal {P} \subsetneq \mathcal {C}\)  with \(\mathcal {P} \ne  \emptyset \)  and suppose that all the following hold:
